1. Field of the Invention
The present invention relates to a method of producing a sensor for monitoring corrosion of the heat-exchanger tubes of a heat exchanger.
2. Description of the Related Art
There are several thousand to several ten thousand of heat-exchanger tubes in a heat exchanger in a thermal power plant. For example, there are more than twenty thousand of heat-exchanger tubes in a condenser in a thermal power generating unit with power of 300 MW. In the most common cases, corrosion or severe corrosion occurs only in a relatively small number of heat-exchanger tubes, while no corrosion or slight corrosion occurs in a relatively large number of heat-exchanger tubes. Leakage resulted from corrosion of heat-exchanger tubes leads to corrosion and scaling of the boiler heating surface, explosion of the boiler tubes and accumulation of salt on a steam turbine in a thermal power plant. As a result, the direct economic losses resulting from the unplanned shutdown of the power generation units and replacing of the water walls of the boiler together with the indirect losses resulting from deterioration of the boiler and the turbine efficiency because of scaling amount up to several million RMB. In a conventional method of producing sensors for monitoring corrosion of heat-exchanger tubes, the sensors are made of the same material as that of the heat-exchanger tubes. However, the sensors made by the above conventional method have the following disadvantages:
1. As there are several thousand to several ten thousand of heat-exchanger tubes in a heat exchanger, corrosion with different degrees occurs in different heat-exchanger tubes of the heat exchanger. However, different corrosion states of the different heat-exchanger tubes can not be monitored by a conventional sensor for monitoring corrosion of heat-exchanger tubes. Especially, a conventional sensor for monitoring corrosion of heat-exchanger tubes can not measure the localized corrosion rate of a corrosion pit in which the most severe corrosion occurs.
2. The corrosion rate measured by a conventional sensor for monitoring corrosion is different from the corrosion rate of actual heat-exchanger tubes. The electrodes of a conventional sensor for monitoring corrosion are made of unused metal material which is the same as that of the heat-exchanger tubes. Although the conditions of the sensors, such as the composition of material, water quality, flow rate of water, heat load and temperature of water, can be regulated to be the same as those of actual operating heat-exchanger tubes, the surface state of sensor is different from that of the actual interior surfaces of the heat-exchanger tubes. In most cases, the corrosion of the heat-exchanger tubes mainly depends on the interior surface state of the heat-exchanger tubes. Therefore, the corrosion rate measured by a conventional sensor for monitoring corrosion is different from the corrosion rate of the actual heat-exchanger tubes.
3. The cost of anticorrosion treatments for heat-exchanger tubes is high. Considering that the thermal power generating unit operates continuously and the conventional sensors for monitoring corrosion can not measure the different corrosion states of the heat-exchanger tubes in which corrosion has occurred, it is difficult to determine whether an anticorrosion treatment is effective or not. As a result, the cost of anti-corrosion treatment is high and appropriate opportunity for performing anti-corrosion treatment of the heat-exchanger tubes is easily to be missed.